1.1 Single mass system
Consider the system of a single mass, supported by a spring and a dashpot, in which the damping is of a viscous character, see Figure 1.1. The
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Figure 1.1: Mass supported by spring and damper.
spring and the damper form a connection between the mass and an immovable base
(for instance the earth).
According to Newton’s second law the equation of motion of the mass is
m
d2u
dt2 = P(t), (1.1)
where P(t) is the total force acting upon the mass m, and u is the displacement of
the mass.
It is now assumed that the total force P consists of an external force F(t), and
the reaction of a spring and a damper. In its simplest form a spring leads to a force
linearly proportional to the displacement u, and a damper leads to a response linearly
proportional to the velocity du/dt. If the spring constant is k and the viscosity of the
damper is c, the total force acting upon the mass is
P(t) = F(t) − ku − c
du
dt
. (1.2)
Thus the equation of motion for the system is
m
d2u
dt2 + c
du
dt
+ ku = F(t), (1.3)